The present invention relates to a liquid crystal display device which is used for a display of OA equipment, AV equipment, and the like, and particularly concerns a liquid crystal display device which is suitable for use outdoors or in an automobile occasionally at a high temperature.
With respect to a specification of a liquid crystal display device required for portable equipment, the demand has been growing for a liquid crystal display device which is thin and lightweight with a wide range of an operating temperature. The operating temperature of the liquid crystal display device generally needs to be set between xe2x88x9220 and 70xc2x0 C. in view of using outdoors or in an automobile.
Further, in order to realize a liquid crystal display device which is thin and lightweight, a liquid crystal display device has been proposed, in which an insulating substrate has a smaller thickness or a plastic substrate is used.
For instance, in such a liquid crystal display device, two insulating substrates are overlaid to each other via plastic spacers such that a display electrode formed by a transparent conductive film opposes a surface, on which an alignment film and the like are stacked, the substrates are bonded to each other via a sealing material placed on a circumference between the substrates, liquid crystal is filled into a gap between the substrates from an inlet formed on a portion of the sealing material, the inlet is filled with a sealing material made of resin and others, and polarizers are disposed outside the substrates.
After filling liquid crystal from the inlet, when the inlet is sealed with a sealing material such as an ultraviolet curing and thermosetting resin, a pressurized sealing method has been conventionally used, in which both surfaces of a liquid crystal cell are pressurized from the outside such that a liquid crystal layer has an even thickness (cell gap) and the sealing operation is performed with the cell gap being set at a predetermined value. This method is performed for preventing an uneven display, which is caused by a difference in cell gaps.
The pressurized sealing method is disclosed in Japanese Unexamined Patent Publication No. 220546/1996 (Tokukaihei 8-220546, published on Aug. 30, 1996), Japanese Unexamined Utility Model Patent Publication No. 33132/1993 (Jitsukaihei 5-33132, published on Apr. 30, 1993), and others.
Japanese Unexamined Patent Publication No. 220546/1996 discloses a technique for setting a cell gap of the liquid crystal display device at a predetermined value by applying an even pressure to the outer surfaces of the substrates so as to expel excessive liquid crystal.
Japanese Utility Model Patent Publication No. 33132/1993 discloses a technique for eliminating a cell gap difference between a display area and a circumference of the display area that has no electrode, by pressurizing merely an area corresponding to the display area.
Meanwhile, as a method for eliminating an irregular display color resulted from a temperature distribution that is caused by heat conduction from a light source or the outside of a driving circuit, Japanese Unexamined Patent Publication No. 292412/1996 (Tokukaihei 8-292412, published on Nov. 5, 1996) discloses a method for partially changing a cell gap of the liquid crystal display device in accordance with a temperature distribution such that dxcex94n (product of an index anisotropy xcex94n and a cell gap d) is substantially even.
In the above liquid crystal display devices, it is possible to realize a uniform cell gap so as to provide an even display at room temperature without causing an irregular display color.
As shown in FIG. 12, a liquid crystal cell 51 does not cause an irregular display color at room temperature; however, an irregular display color 52, in which colors are faded to white, appears at a high temperature (60-70xc2x0 C.) in the center of the liquid crystal cell 51 when voltage is not applied. This phenomenon occurs in a portable liquid crystal display device such as a mobile phone and a PDA (Personal Digital Assistant), that occasionally operate at a low temperature and a high temperature (xe2x88x9220-70xc2x0 C.).
It is understood that an irregular display color appears at a high temperature due to irregular coefficients of thermal expansion, regarding main materials constituting a liquid crystal cell of FIG. 1. Namely, as shown in FIG. 13, the liquid crystal cell 51 has an even cell gap at room temperature, so that no irregular display color appears. However, in the case of a higher atmospheric temperature, the coefficient of thermal expansion of a liquid crystal 53 is larger by one digit than that of a sealing material 54, so that an expansion amount of the liquid crystal 53 is larger than that of the sealing material 54. Consequently, the center of the liquid crystal cell expands upward and downward, and the cell gap results in an irregular display color.
Further, the technic disclosed in Japanese Unexamined Patent Publication No. 292412/1996 is devised for responding to a refractive index xcex94n of the liquid crystal, that fluctuates in accordance with a partial temperature change caused by a certain heat source. In this technique, as shown in FIG. 14, a temperature distribution formed by the heat source is projected in advance and a cell gap thickness is changed in a target area. Therefore, this conventional art does not solve the influence of heat, that is caused by a change in temperature and is exerted upon the entire liquid crystal cell 51. Thus, the center of the liquid crystal cell 51 expands upward and downward and a cell gap difference results in an irregular display color.
An object of the present invention is to provide a liquid crystal display device for preventing an irregular display color which appears in accordance with an ambient temperature varying from room temperature to a high temperature.
In order to solve the above problem, the liquid crystal device of the present invention, that includes a pair of insulating substrates bonded via a sealing material, and liquid crystal filled between a pair of the insulating substrates, is characterized in that a cell gap is formed so as to gradually increase from the center to an end of a display area at room temperature.
With this arrangement, a cell gap is formed so as to gradually increase from the center to an end of a display area at room temperature in a range that no irregular display appears. Hence, it is possible to smooth out a difference in thermal expansion between the liquid crystal and a sealing material when an atmospheric temperature rises, and it is possible to prevent a cell gap from being too large in the center of the display area. Consequently, an irregular display color can be eliminated.
The liquid crystal display device of the present invention is effectively used for an STN liquid crystal display device, in which an operating temperature ranges from room temperature to a high temperature. Particularly, the aforementioned problem frequently appears at a high temperature. Therefore, a cell gap is formed so as to gradually increase from the center to an end of the display area at room temperature in a range that no irregular display color appears, so that it is possible to smooth out a difference in thermal expansion amounts between the liquid crystal and the sealing material at a high temperature. Consequently, an irregular display color can be prevented at a high temperature.
Here, in the present invention, a high temperature is, for example, an atmospheric temperature of about 60 to 70xc2x0 C., and room temperature is an atmospheric temperature of about 25xc2x0 C.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.